ThermalControl — AOSP Vendor Service for Raspberry Pi 5

A full-stack AOSP vendor component that exposes CPU temperature and fan control on a Raspberry Pi 5 running Android 15. Built entirely inside vendor/myoem/ with no modifications to frameworks/ or device/.

Demo

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Architecture

The project follows a 4-layer AOSP vendor stack:

┌─────────────────────────────────────────────────┐
│  ThermalMonitor App  (Kotlin + Jetpack Compose) │
│  apps/ThermalMonitor/                           │
└──────────────────┬──────────────────────────────┘
                   │  Java Binder (ServiceManager)
┌──────────────────▼──────────────────────────────┐
│  ThermalControlManager  (Java SDK library)      │
│  libs/thermalcontrol/                           │
└──────────────────┬──────────────────────────────┘
                   │  AIDL / Binder IPC (NDK)
┌──────────────────▼──────────────────────────────┐
│  thermalcontrold  (C++ native service)          │
│  services/thermalcontrol/                       │
└──────────────────┬──────────────────────────────┘
                   │  C++ interface call
┌──────────────────▼──────────────────────────────┐
│  libthermalcontrolhal  (C++ HAL library)        │
│  hal/thermalcontrol/                            │
│  reads/writes sysfs on RPi5 hardware            │
└─────────────────────────────────────────────────┘

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Layers

HAL (hal/thermalcontrol/)

• libthermalcontrolhal — cc_library_shared built with Soong
• Reads CPU temperature from /sys/class/thermal/thermal_zone0/temp
• Controls fan PWM via /sys/class/hwmon/hwmonN/pwm1 and pwm1_enable
• Dynamically discovers the hwmon sysfs index at init (not hardcoded)
• Supports read/write/auto mode; degrades gracefully when hardware is absent

Service (services/thermalcontrol/)

• thermalcontrold — cc_binary registered as a @VintfStability AIDL service
• AIDL interface: IThermalControlService (NDK backend, libbinder_ndk)
• Launched via init.rc; managed by Android's init process
• SELinux policy (thermalcontrold.te, service_contexts, file_contexts)

Manager Library (libs/thermalcontrol/)

• thermalcontrol-manager — java_sdk_library
• Wraps the AIDL stub with safe defaults and RemoteException handling
• Exposes a clean Java API: getCpuTemperatureCelsius(), setFanSpeed(int), etc.

App (apps/ThermalMonitor/)

• Jetpack Compose UI with real-time polling (2 s interval via StateFlow)
• Displays CPU temperature with color-coded heat category (Cool / Warm / Hot / Critical)
• Fan control: On / Off / Auto mode / speed slider (0–100%)
• MVI pattern: UiState, ThermalViewModel, stateless ThermalScreen composable

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Build Targets

lunch myoem_rpi5-trunk_staging-userdebug

m libthermalcontrolhal          # HAL shared library
m thermalcontrold               # native service binary
m thermalcontrol-manager        # Java SDK library
m ThermalMonitor                # Compose app
m VtsThermalControlHalTest      # HAL unit tests
m VtsThermalControlServiceTest  # VTS binder tests

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Testing

Test suite	Type	Target
VtsThermalControlHalTest	GTest (native)	HAL sysfs read/write, boundary checks, graceful degradation
VtsThermalControlServiceTest	VTS / GTest	VINTF compliance, binder IPC, error codes, concurrency
ThermalControlManagerTest	JUnit (Java)	Manager null-binder safety, range validation
ThermalScreenTest	Compose instrumented	UI state rendering, button interactions, state transitions

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Key Technical Details

• AIDL backend: NDK (libbinder_ndk) — required for vendor processes in AOSP 15
• VINTF stability: @VintfStability annotation with frozen API snapshot (aidl_api/)
• SELinux: custom thermalcontrold domain; hwmon files use generic sysfs label (not sysfs_hwmon, which does not exist in AOSP 15 base policy)
• hwmon discovery: scanned dynamically at startup since the hwmon index is not stable across kernel versions or boot order
• RC file: chown root system + chmod 0664 on hwmon sysfs nodes at on boot — kernel creates them as root:root 0644, blocking writes from the system user

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Environment

Item	Value
Device	Raspberry Pi 5
AOSP branch	android-15.0.0_r14
Build system	Soong (Android.bp)
Languages	C++17, AIDL, Java, Kotlin